This invention relates to the field of pressurized container-dispensers.
Pressurized container-dispensers, often referred to as "aerosols", are very popular and are used in great numbers because of several important advantages including: (1) the convenience they provide in dispensing a wide variety of products; (2) their ability to deliver desired product concentrations; (3) their ability to deliver product in the optimum form for effectiveness in use; (4) their ability to deliver product at a desired rate; (5) their resistance to contamination by virtue of their hermetic seals; and (6) their improved safety, in comparison with many other packaging forms, from harmful misuse by children. Such devices are available in a wide variety of forms. Numerous systems, packages and propellents, and numerous filling and pressurizing methods have been developed. Much effort has been expended on improvement and innovation in this field.
By far the most popular type of pressurized container-dispenser utilizes a condensible gas as propellent. As used herein, the term "condensible gas" refers to a material which is in the liquid phase at the elevated pressures in the container (typically about 15 to 150 psig) throughout the range of temperatures encountered (typically about 30.degree. to 130.degree. F.), but which has a low boiling point at atmospheric pressure. The liquid propellent is charged into the container where it commingles with the product to be dispensed. When the container is sealed, a portion of the propellent evaporates into the headspace (i.e., the space within the container above the fluid product), building pressure in the container until the steady state is reached. As the contents including the propellent are dispensed, the remaining liquid propellent quickly vaporizes to maintain container pressure substantially constant.
Condensible gases have several well-recognized disadvantages as propellents. With some products, the commingling of product and propellent poses a problem in product use. More importantly, there are problems or potential problems inherent in the condensible gases themselves. For example, the popular fluorocarbon propellents have been subject to recent criticism because of a new theory which states that fluorocarbon gases from aerosol containers have an destructive effect on the ozone layer of the atmosphere, which in turn causes an increase in the level of harmful solar radiation reaching the surface of the earth. This potential problem has led the assignee of this application to discontinue further use of fluorocarbon propellents until such time as this theory may be shown to be incorrect. Another group of condensible gas propellents, the hydrocarbon propellents, are flammable under certain conditions. Although such propellents are completely safe when properly used, severe misuse can cause accidents.
Another group of gaseous propellents, the group to which this invention most directly applies, are the "non-condensible" gases, that is, gases which are generally non-condensible in the temperature and pressure ranges typically used or encountered in pressurized packages. Carbon dioxide, nitrous oxide, nitrogen and the inert gases are examples. Since these gases do not undergo a phase change when used in pressurized packages, they are subject to Boyle's law; that is, for a given amount of gas at a constant temperature their pressures are inversely proportional to the volumes in which they are contained. Actually, since such propellent gases are soluble to some extent in the liquid products (i.e., the "intermediates") with which they are used, Boyle's law may not be rigorously applied. However, with most intermediates and with typical initial gas volumes, an acceptably high initial pressure may drop to unacceptably low levels as the product is dispensed and the headspace volume increases. In any case, the pressure drop is severe and makes achievement of uniform dispensing characteristics difficult or impossible with most products, particularly those which do not have a high solvency for the propellent gas.